1. Field of Invention
The present invention relates in general to coated glass fiber substrates for use in making printed circuit boards (PCBs). More particularly, the present invention relates to preventing conductive anodic filament (CAF) growth in PCBs through an enhanced substrate that includes a hydrophobic silane coating of a silane composition intermixed with a silane coupling agent applied to a glass fiber substrate.
2. Background Art
The basic concept behind a coupling agent is to join two disparate surfaces. In the case of printed circuit boards (PCBs), a silane coupling agent is often used to join a varnish coating (e.g., an epoxy-based resin) to a substrate (e.g., glass cloth) to define a laminate, or laminated structure. The silane coupling agent typically consists of an organofunctional group to bind to the varnish coating and a hydrolyzable group that binds to the surface of the substrate. In particular, the alkoxy groups on the silicon hydrolyze to silanols, either through the addition of water or from residual water on the surface of the substrate. Subsequently, the silanols react with hydroxyl groups on the surface of the substrate to form a siloxane bond (Si—O—Si) and eliminate water.
For the specific case of epoxy-based laminates, the organofunctional group that has been found to exhibit desirable performance based on numerous criteria is vinylbenzylaminoethylaminopropyl and also benzylaminoethylaminopropyl. Silane coupling agents, which include this organofunctional group, are thought to covalently bond to the epoxide functional groups of the traditional epoxy-based resin, such as the well known FR4 epoxy resins, through the secondary nitrogens of the amino groups. While a plethora of silane coupling agents exists, the industry workhorse for coupling epoxy-based resins has been vinylbenzylaminoethylaminopropyl-trimethoxysilane (commercially available as Dow Corning Z-6032).
The PCB industry has recently migrated away from the traditional FR4 epoxy based resins (due to lead-free requirements and the higher soldering temperatures associated with tin-silver-copper alloys). Hence, current varnish coatings are typically no longer comprised of FR4 epoxies, rather they are more likely to be bismaleimide triazine (BT) resins or polyphenylene oxide/trially-isocyanurate (PPO/TAIC) interpenetrating networks. Typically, vinylbenzylaminoethylaminopropyltrimethoxysilane, originally developed for traditional FR4 epoxies, is still the coupling agent utilized to couple, or bond, the glass cloth substrate to the laminate varnish. However, other silane coupling agents have been proposed for use in making high-temperature PCBs. For example, U.S. patent application Ser. No. 12/694,005, to Gelorme et al., entitled “SILANE COUPLING AGENTS FOR PRINTED CIRCUIT BOARDS”, filed Jan. 26, 2010, which is a continuation-in-part of U.S. patent application Ser. No. 12/391,500, to Gelorme et al., entitled “SILANE COUPLING AGENTS FOR PRINTED CIRCUIT BOARDS”, filed Feb. 24, 2009, discloses such silane coupling agents, including diallylpropylisocyanuratetrimethoxysilane.
One problem experienced with PCBs is conductive anodic filament (CAF), which results from copper dissolution that emanates from the anode of a circuit and “grows” subsurface toward the cathode, frequently along separated glass fiber/varnish coating interfaces. With PCBs, anode/cathode pairs are typically plated through holes. CAF formation causes a number of reliability issues and can result in catastrophic failure of PCBs, which in some instances can cause fires. The bond between the varnish and substrate is understood to be an important factor in CAF, as is the presence of water in the varnish/substrate interface.
Generally, in locations on PCBs where there are sources of copper, an electrical bias, glass fiber, and moisture, potential exists for the formation of CAF. Typically, CAF occurs at the interface where the glass fiber has delaminated from the varnish, which creates a path for water diffusion. The reason this path is commonly associated with CAF formation is due to the exposure of surface silanols on the glass fibers. Silanols always exist on the surface of the glass fiber and, thus, there is always a pathway for the formation of CAF. Delamination does not have to occur to create this pathway. Additionally, CAF can occur from pre-existing water adsorbed onto the surface of the glass fiber (i.e., water may be deposited during processing of the glass fibers).
Surface silanols, as mentioned above, always exist on the surface of the glass fiber. These surface silanols are reacted when silane coupling agents, such as vinylbenzylaminoethylaminopropyltrimethoxysilane or diallylpropylisocyanurate-trimethoxysilane, are utilized to couple, or bond, the glass cloth substrate to the laminate varnish. As noted above, the alkoxy groups on the silicon of the silane coupling agent hydrolyze to silanols, either through the addition of water or from residual water on the surface of the substrate. Subsequently, the silanols react with hydroxyl groups on the surface of the substrate to form a siloxane bond (Si—O—Si) and eliminate water. Unfortunately, residual alkoxy groups on the silane coupling agent hydrolyze and create more silanols. Thus, more surface silanols are created by the silane coupling agent.
For example, when diallylpropylisocyanuratetrimethoxysilane is used as the silane coupling agent, even though the propyl group to which the trialkoxysilane is attached is hydrophobic (retarding ingress of water to the resin/glass interface and improving CAF resistance), surface silanols are nonetheless created. These surface silanols still provide a hydrophilic path for water diffusion, which leads to CAF formation.
Prior solutions to prevent CAF have typically used the addition of surface modifiers such as n-octyltrimethoxysilane and 3-methacryloxypropyltrimethysilane, but the addition of these silanes requires CO2 reactors which operate at high pressures that are typically unsafe for large scale production. Additionally, the silanes used in this process create silanols, which still provide a hydrophilic path for water diffusion that ultimately leads to CAF formation. Although this process does reduce the likelihood of CAF formation, CAF inevitably occurs nonetheless due to the silanes used as surface modifiers.
Other prior solutions to prevent CAF using coated fiber strands are set forth in U.S. Patent Application Publication No. 2002/0058140 A1, to Dana et al., entitled “GLASS FIBER COATING FOR INHIBITING CONDUCTIVE ANODIC FILAMENT FORMATION IN ELECTRONIC SUPPORTS”, published May 16, 2002. For example, the abstract of the above-referenced published patent application describes the use of a resin compatible coating composition on the surface of glass fibers, the resin compatible coating composition comprising (a) a plurality of discrete particles comprising a silicate having a high affinity for metal ions; and (b) at least one film-forming material. As disclosed in the above-referenced published patent application, particles containing copper getters are imbedded in a polymeric coating. Hence, this solution attempts to prevent the migration of copper through the polymeric coating containing the particles. However, this solution does nothing to address the moisture at the glass fiber/resin interface. By not treating this aspect, a path for CAF still exists. Additionally, the particles imbedded into the polymer coating will never come into intimate contact with the glass fiber and, thus, these particles will not effectively minimize copper migration. Also, there would be a finite amount of copper which could be gettered before the copper getters would become saturated with copper and no longer act as copper getters. Reaching this saturation point would create yet another path for CAF formation.
In another solution disclosed in the above-referenced published patent application, the particles in the resin compatible coating composition can also be formed from hydrophobic polymeric materials to reduce or limit moisture absorption by the coated strand. These particles are contained within a secondary layer which is applied over a primary layer of a primary sizing composition (silane coupling agent). Even though the particles in the secondary layer can be formed from hydrophobic polymeric material, surface silanols are nonetheless created by the underlying primary layer. These surface silanols still provide a hydrophilic path for water diffusion, which leads to CAF formation.
Moreover, the solutions disclosed in the above-referenced published patent application would be plagued with several other problems such as being thick (several microns), exhibiting a different thermal expansion than glass fiber, creating another interface for delamination to occur, severely changing glass cloth manufacturing, as well as being costly and complicated in that a number of materials are required just to prepare the resin compatible coating.
Therefore, a need exists for an enhanced mechanism for preventing conductive anodic filament (CAF) growth in printed circuit boards (PCBs).